Technical Abstract: The effect of high temperature on starch accumulation, starch granule populations, and expression of genes encoding key enzymes for starch biosynthesis was examined during grain development in wheat (Triticum aestivum L. cv. Butte 86). High temperature applied from anthesis to maturity reduced the duration of starch accumulation. Starch accumulation ceased approx. 6 days earlier for grain produced under a 37/17°C (day/night) regimen and 21 days earlier under a 37/28°C (day/night) regimen than for grain produced under a 24/17°C (day/night) regimen. Compared to the 24/17°C regimen, starch content was approx. 19% less for mature grain produced under the 37/17°C regimen and 58% less under the 37/28°C regimen. The smaller type B starch granules were the predominant class in mature grain produced under the 24/17°C and 37/17°C regimens, based on relative volume, whereas the larger type A granules were predominant in grain produced under the 37/28°C regimen. Under the 24/17°C regimen, steady state transcript levels for ADP-glucose pyrophosphorylase, starch synthases I, II, and III, granule-bound starch synthase, and starch branching enzymes I and II were highest from 12-16 days post-anthesis (dpa). Under the 37/17°C regimen, steady state levels of these transcripts followed the same temporal pattern, but were substantially lower. Under the 37/28°C regimen, transcript levels peaked earlier, at 7 dpa. The high temperature regimens reduced the relative levels of transcripts for starch synthase more than the other starch biosynthetic enzymes.